The present invention relates in general to improved apparatus and methodology for peristaltic pumps and in particular to improved pump roller drive mechanisms therefor resulting in improved pump tubing life and fluid delivery rate accuracy.
The basic design of typical conventional peristaltic pumps has been well-known and widely used to good advantage for many years. Such basic conventional design involves a length of pump tubing through which fluids to be pumped are received. Such tubing is typically resilient and pliable, and intended to be engaged by a plurality of pump rollers as the tubing is otherwise engaged against a rigid, curved backplate surface.
The pump tubing itself is regarded as being a consumable item, intended to be replaced as it "wears out."
Peristaltic pumps have been especially useful for many years in applications requiring relatively low fluid delivery rates and/or at relatively low pressures. Also, isolation of the fluid to be pumped in a pump tubing, generally without access thereto, helps prevent contaminating either the fluid or the pump itself. Such characteristic of isolated pumping ability is uniquely usable in certain applications, for example, if the material being pumped is chemically reactive or otherwise inherently dangerous.
Generally speaking, per the basic conventional design, fluid to be pumped enters one end of the pump tubing and is then advanced by progressive compression of the tubing between the rollers and the backplate surface. In essence, the fluid is advanced by being trapped in incremental amounts in the tubing between adjacent pairs of rollers, until it is forced through the entirety of the tubing by the action of the rollers and is expelled from an output end of the pump tubing.
Peristaltic pumps are generally very reliable due to their inherent simplicity. The fluid delivery rate itself is readily controlled through use of precision variable-speed electric drive motors.
Though generally simple in its basic design, peristaltic pumps are basically precision instruments of relatively higher costs, for example, costing possibly as much as $2000 to $2500 each.
The cost factor alone precludes use of peristaltic pumps in some applications, particularly where fluid isolation or delivery rate accuracy is not critical. However, the combination of their generally high reliability and the ease of flow rate control in otherwise demanding environments has resulted in the relatively wide spread use of peristaltic pumps in a number of stringent demand applications, such as for sample introduction into analytical instruments (e.g., ICP, DCP, Atomic Absorption, and the like), for the introduction of pharmaceuticals into intra-venous supply lines, and for the transfer of blood and/or other biological fluids. Peristaltic pumps are also often used for the introduction of fluids into chemical reaction vessels or similar arrangements, especially in small test bed or pilot plant operations, where critical controls and measurements are desired.
Despite their generally high reliability and accuracy, the performance of the pump is itself completely dependent on the performance of the pump tubing. "Wearing out" of a length of pump tubing occurs whenever the resilient, flexible pump tubing has excessively stretched due to its use. It is to be understood that the reaction of the rollers against the pump tubing is what actually performs the pumping work, and is also the source of the forces having a tendency to stretch the tubing during use. As a result of such stretching, the tubing internal diameter can be literally reduced in areas. Accordingly, the volume of fluid trapped between adjacent rollers can be correspondingly reduced.
In other words, in the face of such stretching, while the pump drive motor continues to operate at a highly accurate rate of turn, the progressive reduction in the tubing diameter and the consequential reduction in tubing volume between respective pairs of rollers, results in a progressive reduction in fluid delivery rate.
The above tube stretching phenomenon is therefore a significant drawback of typical conventional peristaltic pumps.
Another aspect of such phenomenon is that the progressive reduction in peristaltic pump delivery rate is particularly pronounced at relatively higher delivery rates. In other words, as a pump is run at relatively higher speed, such as between samples (to reduce instrument down-time between samples), the rate of undesired tube stretching increases.
Still another aspect of the undesired tube stretching phenomenon relates to the nature of the tubing material itself. In conventional practices, different tube materials are utilized for different applications. For example, silicone and fluoropolymer types of tubing may be typically used for chemically reactive and corrosive fluid pumping. However, such materials are relatively soft and mechanically weak, making them particularly susceptible to stretching damage. Such factor is especially a problem given the relatively higher costs of such types of tubing. Relatively less expensive vinyl tubing is generally less susceptible to stretching degradation (though stretching damage still occurs over time), but is not usable for certain applications, such as are the silicone and flouropolymer types of tubing, for handling particular fluids and/or operating in particular environments.
The bottom line for all basic designs of conventional peristaltic pumps is that there is a frictional drag between the pump rollers and the pump tubing, which results to a lesser or greater degree in undesired tube stretching. While the rate of such stretching varies with materials and/or pump operational speeds, the replaceable tubing (of basically any used material) is susceptible to such stretching damage, with commensurately reduced tubing life and degraded pump delivery rate accuracy.